The origin of Devils Tower has been a matter of
speculation for many years, and even today after detailed geologic
mapping of the area, no conclusive proof of its mode of origin can be
presented.

Several theories of the origin have been proposed.
One of the more popular of these is that it is the neck of an extinct
volcano (Carpenter, 1888; Dutton and Schwartz, 1936). Another theory
is that Devils Tower and Missouri Buttes (a mass of the same type of
rock about 4 miles northwest of the Tower) are the remnants of a laccolith
(a tabular intrusive igneous body, thickest in the middle, and
with a relatively level floor), the vent for which was under Missouri
Buttes (Jaggar, 1901, p. 264). Darton (1901, p. 69) believed that the
Tower is the remnant of a laccolith, smaller than the one proposed by
Jagger, the feeding vent for which was underneath the Tower.

Much more detailed geologic work will have to be done
in the surrounding area before the mode of origin of Devils Tower may be
proved conclusively. The evidence gathered during the present
investigation, however, suggests that Devils Tower is a body of intrusive
igneous rock, which was never much larger in diameter than the present
base of the Tower, and which at depth (1,000 feet or more) is connected
to a sill or laccolith type body. The bases for this theory
are

1. The exposed portion of the Tower is the result of
recent erosion. At the time of its intrusion it was surrounded and
probably covered by several hundred feet of sedimentary rock.

2. The mineral composition and texture are more
typical of shallow intrusive rocks, which are formed at depth, than
extrusive rocks, which are formed on the surface.

3. No evidence of extrusive igneous activity has been
found in the surrounding area.

4. Missouri Buttes, about 4 miles to the northwest,
and the Tower have the same composition so it is assumed that they were
derived from a common magma; possibly the magma of a large intrusive
body, such as a laccolith or sill.

5. In a well drilled about 1-1/2 miles southwest of
Missouri Buttes, near the center of a structural dome, rock similar to
the Tower and Missouri Buttes was encountered at about 1,400 feet below
the base of Missouri Buttes. Inasumuch as the thickness of the
sedimentary rocks in this area is normally much greater than this depth,
the rock in the drill hole probably represents an intrusive body, rather
than the Precambrian igneous rocks upon which the younger sedimentary
rocks were deposited.

6. The relatively small amount of talus, slope wash,
or terrace gravel derived from the Tower and Missouri Buttes suggests
that they have not been extensively eroded, and therefore the original
igneous bodies were not much larger than at present.

7. Columnar jointing is common in intrusive bodies
formed at comparatively shallow depths.